There isn't a certain number that is correct for photos. A few from each visit together is generally fine. Photos are just one means of showing the development of your relationship. For example: 2 pics with the grandparents is just as good as 10.
Print out a few per page so that you can put a caption below. Or, if you're sending snapshots, make black and white copies with a few per page and then have the original photos (at the interview) in a plastic bag with the captions written on the back.
